Hello @Sam_Schumpf,
At the moment you can already set a due date for subtasks that are added via rule:
Now if you want the due dates to change based on the parent task you would need to set up dependencies which currently cannot be set up via rule for subtasks, however they could be set up when using task templates.
Or otherwise check out Flowsana.
And you can also upvote here.
Here is another feedback request thread you might be interested in.